A Massive Volcano Beneath Europe Is Stirring

And millions of lives may be at risk.

Seen from mountaintop monastery Hermitage dei Camaldoli, the suburban sprawl of greater Naples sits atop a massive volcanic caldera that may be poised to erupt. Posillipo Hill, the dark ridge on the left, is part of the wall of the caldera, which stretches 12 kilometers across.

Russ Juskalian

Just 10 kilometers from the frenetic pulse of central Naples, in stark contrast to the Italian city’s impressive volcanic-stone churches and effortlessly stylish urbanites, sits a boxy, concrete building. Inside this unremarkable government outpost, accessed through a pair of sliding glass doors, is the Vesuvius Observatory monitoring room, lit by the cool glow of 92 flat-panel screens. On each screen, volcanic activity readings, including those from seismic devices sensitive enough to pick up a passing bus, blink and beep in real time. In the middle of the room is a desk. And in the middle of that desk is a single red phone.

Twenty-four hours a day, 365 days a year, there are at least two people in the room, ready to pick up the phone and advise the national civilian defense in the event of a volcanic emergency.

But Mount Vesuvius, its iconic cone rising conspicuously on the city’s eastern flank, is not the only concern. A potentially even more destructive volcanic giant is tossing fitfully in its sleep, right on Naples’ doorstep: the caldera of the massive volcano system Campi Flegrei, which translates to the fields of fire.

Dozens of monitors at the Vesuvius Observatory outside Naples track earthquakes and other regional volcanic activity. The observatory’s director, Francesca Bianco, points out recent tremors around the facility’s namesake, but the bigger threat to the metropolis may be Campi Flegrei, a massive caldera beneath the suburbs, seen in the upper left monitor.

Russ Juskalian

If it erupts, an event some researchers feel is increasingly likely, it could be catastrophic for Italy’s third-largest municipality and the surrounding countryside. Disruptions could stretch far beyond Italy, too, affecting everything from air travel to agriculture, with ash darkening the skies over Europe and the Mediterranean.

The threat comes from the west, in a pockmarked and mountainous landscape abutting Naples just beyond an elongated ridge thick with lovely villas, called Posillipo Hill. The meaning of the hill’s name, “a respite from worry,” belies the story of its formation. Posillipo is at the edge of a volcano caldera so large that to see its full shape requires an elevated vantage point. To stand within it is to be unable to see it.

These calderas are born when a volcano system erupts with such force that the resulting crater, instead of merely being flattened, actually slumps downward into the ground afterward. The most powerful eruption believed to be from Campi Flegrei, nearly 40,000 years ago, launched the equivalent of 300 cubic kilometers of ash and pulverized rock skyward. The massive eruption impacted the global climate and may have helped to snuff out the last gasps of the Neanderthals.

Now there are signs that Campi Flegrei is stirring once more.

At surface level, the caldera is dotted with steam vents, or fumaroles. One of them, the Solfatara di Pozzuoli, has famously lent its name to fumaroles that emit sulfur — such vents around the world are now known as solfataras. But it was one of Solfatara’s less well-known neighbors, the fumarole Pisciarelli, that attracted attention in 2009. The once-insignificant Pisciarelli started to roar, bubbling mud and spewing steam. It was a hint that something was happening below ground.

Volcanologist Giovanni Chiodini (in red) shows a visitor around the edges of the Pisciarelli fumarole, an increasingly active volcanic vent within the Campi Flegrei caldera.

Russ Juskalian

In 2012, the land within the caldera, which had been rising for nearly a decade, began to rise faster. And in late 2016, a paper in Nature Communications suggested the volcano might be entering a new and potentially much more dangerous phase.

There’s just one problem: The dynamics of large calderas are at best a mystery, and reconstructing the steps that led to Campi Flegrei’s past eruptions seems as much Delphic interpretation as science. Experts can’t agree on what the volcano is doing, only that the threat it poses is real.

In the Air

Campi Flegrei is unique among volcanoes in that it can be reached by subway. In fact, from the trendy beachfront neighborhood of Chiaia in Naples, it’s just two stops to the caldera. “Do you smell that?” says my friend Emanuel Scholz, a German geologist who has joined me in the caldera out of professional curiosity on a mild February morning. It smells like sulfur.

“This is Campi Flegrei,” says volcanologist Giovanni Chiodini, waiting for us on the subway platform. “Some people think only the Solfatara is the volcano because there is steam coming out. But this is all a volcano.” He moves his arms in a full circle, indicating the entire urban outskirts around us. Then he points down the road toward the Vesuvius Observatory. It’s in the caldera, too, he notes.

Chiodini, Scholz and I pile into an underpowered van, and we fight our way through winding streets of molasses-slow traffic. Our destination is the Pisciarelli fumarole, on a small hillside just meters behind an artificial soccer field. (The suburban banality of the setting is surreal. I imagine an irate gym teacher with his hand on his forehead as he loses yet another ball to the volcano.)

Geologist Emanuel Scholz and an observatory staff member chat beside the Pisciarelli fumarole as it spews gas and boiling mud just a ball’s toss away from a suburban soccer field.

Russ Juskalian

As we approach the fumarole, white plumes of caustic vapor bite at the insides of my nostrils. I hear the guttural roar of what sounds like a redlining motor. “Some years ago,” says Chiodini, “This was a trickle. Not like today.” Dark, gray mud bubbles violently from the fumarole’s center in fist-sized globules and snakes downhill toward the suburb’s homes. The whole thing has a dank and foreboding air.

It’s here and at Solfatara that Chiodini believes he’s found the signature of a waking volcano. As the lead author of the 2016 Nature Communications paper, Chiodini documented a change in the molecular makeup of gases spewing from the fumaroles, particularly Solfatara, suggesting that the Campi Flegrei caldera might be approaching the so-called “critical degassing pressure” (CDP), after which an eruption becomes far more likely.

His argument hinges on the fact that as magma rises through Earth’s crust, it undergoes a process called decompression, during which it releases a variable mix of volatile compounds. At the CDP, this mix switches over almost fully to water vapor. The massive amounts of water-rich gases then heat hydrothermal systems in the surrounding rocks. The result: at least a tenfold increase in heat transported into the rock layers between the magma and the crust’s surface, weakening them.

With a slight shift in wind, a cloud of gas from the fumarole engulfs Chiodini .

Russ Juskalian

Up on a small outcrop above the main vent of the Pisciarelli fumarole, the observatory has two monitoring systems looking for changes in heat and gas composition emerging from the caldera. One, tucked beside solar panels and under a small shelter, is little more than a coffee-filter sized dome. Every two hours, it measures the gases seeping up from the ground. The other is an infrared camera pointing at an even higher outcrop. The heat moving into the rock there, Chiodini explains, might be a good measure of how active the caldera is.

Similar monitoring stations are scattered across the caldera. And what they’ve recorded is concerning: a 25-year decreasing trend in the ratio of certain gases suggests that decompression is occurring and magma may be rising closer to the surface, while an uptick over the last 15 years in heat transfer matches Chiodini’s model of what CDP will look like.

The data from the caldera parallels similar trends found before eruptions at smaller volcanoes in Papua New Guinea and the Galapagos. And, according to Chiodini, it suggests that Campi Flegrei’s magma is preparing to let off a dangerous amount of heat. For a while the caldera floor, sitting at ground level, and the rock below it, will act like a plug, holding it all together. But keep adding heat and weakening the rocks, and the plug will eventually fail. Perhaps catastrophically.

In the Rock

Giuseppe De Natale, a physicist by training who monitors the volcanoes at the Vesuvius Observatory, had access to the same data as Chiodini. But his conclusions were very different.

Changes in measures such as heat transfer and gas composition are not necessarily red flags; De Natale and colleagues noted in a Nature Communications paper of their own, published in May, that the majority of episodes of such unrest in a large volcano system do not lead to eruption.

The Heat Is On

On the edge of Naples, the Campi Flegrei caldera, a massive volcano, appears to be stirring. One researcher believes the caldera’s magma may be releasing massive amounts of hot gases, which pump heat into an overlying hydrothermal network of water and rock. This added heat weakens the rocks between the magma chamber and ground level — and may make an eruption much more likely.

Jay Smith

The real worry, De Natale believes, is not decompressing magma but accumulating stress on Earth’s crust. To find signs of impending danger, look to the ground, De Natale tells me: The caldera’s narrative was written into the land’s deformation over millennia.

The long record comes courtesy of the nearly 2,000-year-old columns amid ruins of an old Roman marketplace still standing near the waterfront of a nearby town called Pozzuoli. Discovered in 1750, the columns puzzled scientists: They’re dotted with boreholes drilled by a marine mollusk known as Lithodomus lithophagus. It took 200 years to solve the riddle. The so-called “stone-eater” mussels had done their work as the columns moved into and out of the water with the fluctuations of the caldera floor’s elevation.

By dating the holes, scientists reconstructed a remarkably consistent record: The ground in the caldera had been sinking at a steady rate of about 1.7 meters per century. That is, except for brief periods when the caldera floor had risen. This happened twice in the past 500 years. The first started about a century before the last minor eruption in the caldera in 1538, which gave birth to a 134-meter mountain. The second is what volcanologists are worrying about today.

In his office, De Natale shows me a simple line graph of the current rise. The most notable features are sharp uplifts of 1.7 meters and 1.8 meters, respectively, from 1969 to 1972 and 1982 to 1984. The latter uplift period was accompanied by numerous small earthquakes — including 600 on the worst day in 1984. On charts tracking the rate of movement of the caldera floor, the uplifts, which occurred in mere geological instants, appear as nearly vertical lines. “But look,” says De Natale excitedly, “the period of uplift from 2005 onward is totally different.” Unlike the previous instances, here the line only gradually bends upward.

De Natale’s explanation is that the two uplift periods of the 1970s and ’80s were likely caused by magma rising from a chamber 8 kilometers underground to form a shallower sill about 4 kilometers beneath the city. As the crust strained to contain the pressure, it fractured, causing the earthquakes. But then the magma stopped moving, and, sometime around 2000, the shallower layer of magma had almost completely cooled.

From 1985 until around 2000, the caldera slumped by nearly a meter. Since 2005, the current slow uplift has recouped much of that loss in elevation, but with less seismic activity than in previous periods.

One view, says De Natale, is to take the uplift episodes of the 1970s, 1980s and now as independent events. The first two had deformed the ground by more than twice the uplift currently underway, so surely there wasn’t anything yet to worry about.

But what if, De Natale says, the three periods are all connected? In that case, the caldera floor is a lot like the proverbial camel’s back: An overweight rider might cause the poor animal’s spine to arch without lasting damage. But load the camel with enough weight, and eventually even the lightest additional cargo — perhaps a single straw — will result in disaster. In a similar way, the breaking point of the caldera floor might not be determined by any single displacement, but by the total net displacement since the process started.

If this view holds, the risk for an eruption isn’t determined by the modest uplift since 2005, but by the nearly 4 meters of uplift since 1950. That would mean the caldera is already under considerable tension. The question is how much more it could take. Even De Natale isn’t ready to say.

Rocks colored vibrant yellow by sulfur emissions near Solfatara.

Russ Juskalian

A Giant’s Footprints

To understand why Campi Flegrei poses such an unpredictable and enigmatic risk requires a removed vantage point and a history lesson. Or, as Antonio Costa, an expert on the formation of calderas tells me, “Without geological history, you cannot know the current situation.” So, after spending a few days with Chiodini and De Natale, Scholz and I join Costa and volcanologist Roberto Isaia at a mountaintop monastery with a view of the caldera and its surroundings.

Near the back of the grounds, a stone terrace opens to the stunning vista of a semicircular valley composed of visible craters in its center — remnants of the caldera’s 70 “small” eruptions in the past 15,000 years — and beyond that the deep blue water of the Gulf of Naples.

As Costa unrolls a topographical map of what we’re looking at, Isaia becomes animated. His fingers trace the ridge of Posillipo Hill, arcing into the ocean where the island of Ischia sits opposite us. Then he traces a line from the other side of the valley, completing an 13-kilometer-wide oval. “That,” he says, “is the volcano.”

The sprawling, disorganized, traffic-bound mess in the middle is teeming with people. We can see Stadio San Paolo, the third-largest stadium in Italy, with a capacity of 60,000. The observatory is somewhere in there, too.

“A new eruption could happen anywhere down there,” says Costa.

From an overlook at the mountain monastery of Hermitage dei Camaldoli, the suburban sprawl stretching across Campi Flegrei is obvious; the entire densely populated area falls within the red zone on a map showing the area of greatest risk should the caldera erupt.

Russ Juskalian

He produces a timeline of Campi Flegrei’s periods of rest and unrest. “We don’t know if this is the start of a new epoch or not,” says Costa, noting the irregularity of its past activity.

One risk probability map of the area we’re looking at, modeled on eruptions from the past 5,000 years, looks like a rainbow of concentric paint splatters, each larger than the next, with finger-shaped bands extending outward from an epicenter in Pozzuoli. Each color represents the annual probability of being hit by pyroclastic flows, gravity-driven currents of superheated rock and debris that move a lot like an avalanche.

The real threat for the greater Naples area, however, is the ash that Campi Flegrei might send skyward. The prevailing wind patterns mean even moderately sized eruptions would drop the bulk of their ejected ash right on the heads of Neapolitans. Drop enough of it, and the flat roofs around the city would start collapsing.

It’s a scenario never far from my mind over the last days of my trip. And as we finally prepare to leave Naples, I turn to Scholz and sheepishly admit that, as irrational as it might be, I feel a sense of relief to be going home.

“Me, too,” he replies.

Postscript or Prologue?

Media coverage of Campi Flegrei’s potential threat erupted around the end of 2016 with the publication of Chiodini’s study. In May, the paper De Natale co-wrote raised alarms again. “Set to Blow?” read one British tabloid. “Italy’s Supervolcano May Be on a Course to Erupt,” warned another. But the caldera itself has been quiet.

“The ground level is stable, and the seismicity almost absent,” De Natale writes via email when I ask what the fuss is about. The paper isn’t so circumspect. It postulates that Campi Flegrei’s crust may have as little as a meter left to give before “an eruption can be expected.”

The team reached this conclusion by scrutinizing the changing patterns of small earthquakes and uplift at Campi Flegrei and analyzing physical markers of stress from a deep drilling program. They also compared the data against that of other volcanoes, notably two vents in the similarly sized Rabaul caldera in Papua New Guinea that erupted simultaneously in 1994. (Thanks to a local preparedness campaign, the eruption killed only four people.)

Their findings offer a model for how rising magma and pockets of hot water or gas stretch the ground beneath a caldera in three distinct phases. In each phase, the ground is less elastic. The final “inelastic” phase is a sign that the crust is stretched to its limits and riddled with small fractures: rigid and ready to erupt.

A monitoring station keeps close tabs on activity in the caldera.

Russ Juskalian

The current uplift, wrote the authors about Campi Flegrei, “suggests that the crust is now approaching the transition from quasi-elastic to inelastic deformation.” The caldera floor has been pushed upward by about 4 meters since 1950. The bad news is that, if their model is correct, the caldera floor should become inelastic at between 5 and 12.5 meters. The volcano’s unpredictable history and our limited understanding of how caldera volcanoes in general work make it impossible to know whether that limit could be reached in years, decades — or never.

That leaves the people of Naples with two models in disagreement over signs of a potential imminent eruption: one from Chiodini, looking for the CDP and heat-weakened rocks, and one from De Natale, focusing on an accumulation of tension in rocks bent to their limits. To laypeople, that might sound like a reason to trust neither. However, there’s one area where the two models do agree: Campi Flegrei is acting in a way that suggests its first reawakening in 412 years.

The long-term life cycle of caldera volcanoes is still largely a mystery, however. Instead of the tossing and turning of a waking giant, both researchers suggest that Campi Flegrei’s movements may simply stop, and the volcano sinks back into a deep sleep. The problem is that nobody will know until it erupts — or doesn’t.

That uncertainty reminds me of the unsettled feeling I had walking the streets of Naples, knowing the chance of an eruption is statistically minuscule, but always there. But it also makes me think of what I like most about Naples: its seemingly endless capacity, no matter the conditions, to keep living.

It’s a city flanked by volcanoes, with magma beneath its feet. Its cathedrals are literally built of volcanic ash. Almost, I think, like an act of defiance.